It is not feasible to conduct randomized controlled exercise intervention trials with breast cancer diagnosis as the primary outcome. Therefore, it is of interest to determine whether exercise will alter physiologic outcomes associated with breast cancer incidence, including oxidative stress, estrogens, estrogen metabolism, and metabolic factors such as body fat, elevated fasting insulin, insulin resistance, alterations in plasma levels of IGF-axis proteins. We propose to examine the effects of aerobic exercise training on each of these mechanisms among young (18-30 years), pre-menopausal, eumenohrreic women in a randomized controlled trial. The primary hypothesis to be tested is whether exercise alters oxidative stress as measured by F2-lsoprostanes. The innovation of this grant stems from the concurrent measurement of cancer biomarkers, which will enable us to explore relationships between changes in these physiologic parameters that may have an important role in the exercise - cancer link. Compelling preliminary data indicate that the amount of exercise recommended for health promotion and chronic disease prevention (5 weekly 30 min sessions of moderate intensity aerobic exercise) may alter these risk factors in a manner consistent with reduced cancer risk. We will recruit 400 women and anticipate a dropout rate of 20%, for a final sample of 320 women (n=160 per group). Measurements will be made from days 7 to 10 of the participants menstrual cycle prior to randomization and at the 5th menstrual cycle after the baseline cycle. Participants will also use ovulation kits to determine whether exercise alters luteal phase length or ovulatory status. Measurements will include oxidative stress (F2-isoprostanes), estrogen metabolites, body composition (dual energy x-ray absorptiometry), insulin, glucose, and insulin resistance (HOMA index), insulin-like growth factor axis proteins (IGF-1, IGFBP-1, -2, and -3), submaximal fitness, questionnaires (injury/illness, demographics), and diet (by food frequency questionnaire).